The invention relates to a method for the separation of methyl mercaptan from reaction gas mixtures obtained by means of catalytic conversion of H2S with methanol.
Methyl mercaptan is an industrially important precursor for the synthesis of the nutritive amino acid methionine, or the hydroxianalogous 2-hydroxi-4-methyl mercaptobutyric acid (MHA), as well as for the production of dimethyl sulfoxide and dimethyl sulfone. Today, it is predominantly produced from methanol and hydrogen sulfide by means of the conversion onto a catalyst of alumina. The synthesis of methyl mercaptan usually occurs in the gas phase at temperatures between 300 and 500° C., and at pressures between 14.5 and 362.59 psi (1 and 25 bar). In order to increase the activity and selectivity of the catalyst, the same is usually coated with potassium woiframate as the promoter. The conversion of hydrogen sulfide and methanol into methyl mercaptan is an exothermic process, in which 28,500 KJ are released per kilomols of converted methanol. One method is described, for example, in EP 0 850 923 B.
In addition to the desired methyl mercaptan, the product gas mixture of the synthesis also contains the water created during the reaction, and as the byproducts, dimethyl sulfide, dimethyl ether, low-amounts of polysulfide, such as dimethyl disulfide, as well as non-converted methanol, excess hydrogen sulfide, and the inert gases nitrogen, carbon dioxide, carbon monoxide, and hydrogen in terms of the reaction. The separation of the product gas mixtures into their components serves for the extraction of methyl mercaptan and dimethyl sulfide, for the discharge of water and inert gas components, as well as for the return of the unused methanol and hydrogen sulfide into the synthesis reactor.
DE-PS 17 68 826 relates to a method for separation, in which the product gas mixture is distilled at a pressure of not more than 159.5 psi (11 bar), and at a temperature of 10 to 140° C. The gaseous phase of this distillation essentially consists of hydrogen sulfide, inert gases, dimethyl sulfide, and methyl mercaptan. Methyl mercaptan and dimethyl sulfide are eluted in counterflow from the gaseous phase with methanol. The remaining hydrogen sulfide and the inert gases are returned to the synthesis reactor as cycle gas. The loaded elution methanol is again processed by distillation together with the practically hydrogen sulfide free sump of the distillation, and also returned to the manufacturing process.
An improved method having a higher definition in the separation of the product gas mixtures in the individual flow of the matter is described in EP 0 850 923 B (U.S. Pat. No. 5,866,721).
In addition to the high investment and operating costs (columns of 40 theoretical steps with a nominal pressure level of 232 psi (16 bar) are usually required, wherein the nominal pressure level is a measure for the stability of the reactor used), one disadvantage of this processing by distillation of the complex reaction mixture is the inevitable formation of residue that must be disposed of, and as a consequence thereof, the loss of resources. Additionally, a 2-phase mixture of water and sulfur compounds may occur during processing by means of. distillation of the said reaction mixture in the sump, which makes the controlling of the column substantially more difficult. In addition to environmental pollution, the further processing of methyl mercaptan without the separation of the said admixtures, however, may lead to substantial product failures in the production of the subsequent product, such as methionine, thus making separation unavoidable.